Why are angles opposite each other when two lines cross called vertical angles?

Fanny mendelssohn manages all the magazines and paperback books at a local grocery store. she prices many paperbacks to sell at $7.99. the cost to the store is $5.29. what is the markup rate on a paperback book, based on its cost?

In the diagram below? p is circumscribed about quadrilateral abcd. what is the value of x

The ph level of a blueberry is 3.1 what is the hydrogen-ion concentration [h+] for the blueberry